In the observation of Brillouin scatter with a Fabry-Perot interferometer, it is known to scan the spectral region of interest of the scattered radiation by periodically shifting one of two mirrors with a piezoelectric mirror holder to change periodically the spacing of the two mirrors and at the same time to maintain the parallelism of the two mirrors by a control and regulation circuit operating on the holding device of the other of the two mirrors, as disclosed in Vol. 2, No. 2 of "Optics Communications", pp. 73 to 76 (July 1970). The control and regulation system for maintaining parallelism comprises two portions of which one operates on a first piezoelectric holder-displacing device to control the tilt of the mirror about an axis that may be referred to as the x axis and a second portion that operates on a second piezoelectric holder displacing device that controls the tilting of the same mirror about an axis perpendicular to the x axis, referred to as the y axis. The control signal is derived from the observed radiation to be measured coming out of the interferometer and is generated in such a way that during alternating brief periods determined by a timing circuit the mirror serving for control of parallelism is first checked and controlled with respect to one tilt axis and then similarly checked and controlled with respect to the other, in each case being swung very slightly back and forth about the axis in question in order to determine at what position the resulting intensity of a Rayleigh line is maximized in order to generate a positioning signal in the direction (of the sign) which will increase the line intensity.
In a control and regulation system of the kind above described both mirrors must be displaceable by piezoelectric adjusting devices or other devices of similar effect and, consequently, such a system is not suited for application to commercially available piezoelectrically adjustable interferometers in which only one of the usual pair of mirrors is provided with piezoelectric displacing devices and that one is only axially adjustable.
In another known control and regulation system for spacing and parallelism control of interferometer mirrors only one of the mirrors needs to be mounted on piezoelectric devices, that one being mounted on three piezoelectric devices. This known device operates by reference to sampling different points of the Rayleigh line and thus allows regulation of mirror parallelism only if at the same time the axial drift of the interferometer with reference to the light source wave length is regulated in coordination, which leads to a considerable narrowing of the control region for the parallelism regulation and/or for the duration of a measurement. Shifts of the interferometer transmission frequency with respect to the fundamental frequency of an incident beam produced by a laser, such frequency shifts resulting from temperature and air pressure fluctuations, require much greater adjustments for compensation than are necessary for the regulation of mirror parallelism, since the mirror parallelism is affected only by the differential changes produced by such forces as uneven thermal expansion, with respect to the three mirror support points.
It is an object of the present invention to provide an electronic control and regulation system for mirror spacing and parallelism in an optical system such as that of a Fabry-Perot interferometer which will overcome the disadvantages described above.